1. Field of the Invention
This invention relates generally to a class of equipment known as wireless terminals such as mobile devices; and, more specifically, to a method and apparatus for fine tuning the signal path to a pseudo-random number with a new improved tracking circuit in a Code Division Multiple Access (CDMA) wireless infrastructure.
2. Description of the Prior Art
The ever increasing availability and popularity of wireless communications can be linked to technological gains that have provided more efficient, reliable and cost-effective mobile devices, such as wireless digital telephones and personal communication systems. Due to their mobility and low power requirements, conventional mobile devices impose significant design constraints upon the wireless communication networks and, more particularly, the switching offices that support them.
Each switching office is associated with multiple transceiver sites, or cells, that enable communications between mobile devices and the switching office. Typically, there is a high density, or closeness, of cells per geographic area, often in a honeycomb pattern of overlapping cells of communication. Cell density causes each mobile device to always be close to at least one cell. Thus, a wireless signal may be concurrently heard by several cells and, possibly, several switching offices. Each cell generally covers a range of several miles in each direction which may, of course, be limited by natural or man-made objects such as mountains, buildings, etc.
In the past, wireless communications were largely analog based but, in recent years, the wireless carriers have moved toward digital based communications. This transition stems from compatibility and frequency utilization perspectives. If users can share a frequency or a range of frequencies, then more users can be accommodated on less bandwidth.
An increasingly popular wireless digital communication system is Code Division Multiple Access (CDMA), which uses spread spectrum technologies. For example, a transmitter transmits at a first frequency at a first time and at a second frequency at a second time, and a receiver is synchronized to switch frequencies during the reception of these frequencies in response to the change from the first to the second frequency.
Whenever multiple signals are communicated through a communication network, the potential for losing data or degradation of the communication signal may increase exponentially. Maintaining synchronization between a transmitter and a receiver is mandatory. If the synchronization or timing of the transmission or arrival of a signal is off, then the information content of the signal may be distorted or lost. This distortion or loss is commonly referred to as slippage.
Searching for and tracking a communication signal are two of the most important synchronization processes performed by the receiver. The searching process operates to find or locate possible signal paths in order to demodulate the strongest received communication signal as well as to provide candidates for a soft handoff.
The tracking process, in contrast, operates to track a received communication signal. This is often accomplished using a tracking circuit. Conventional tracking circuits work to fine tune the signal path, most often to a static pseudorandom number (PN) chip. In one prior art embodiment, the input signals of tracking circuits are early and late sampled PN despread pilot symbols. If the early and late samples have about the same energy, then the timing is considered to be correct. In another prior art embodiment, the input signals to the tracking circuit are early, late and ontime sampled PN despread pilot symbols signals. The product of the ontime pilot symbol with the difference of the early and late sampled PN despread pilot symbols are used as an error signal to fine tune the sampling time. What is needed is a new improved method for tracking a received signal.
A tracking circuit fine tunes the sampling time to a resolution of {fraction (1/16)}, xc2xc or xe2x85x9 of Tc, depending on the circuit design. In the embodiment disclosed the sampling time is xe2x85x9 Tc, where Tc is a chip duration after the searcher circuit finds the timing to within 0.5 Tc. Depending on the prior art tracking circuit that is being used, the input signals are early and late sampled PN despread pilot symbols or early, late and ontime sampled PN despread pilot symbols. Early and late samples are plus or minus one tick or Tt (one tick is xe2x85x9 Tc) away from the ontime samples. The basic principle is that if early and late samples have about the same energy, then the timing is correct. If not, then the sampling time is changed to obtain a balance. In this invention, improved performance over the prior art tracking circuit is obtained by using pilot estimation results instead of the actual ontime pilot symbol signals. In one embodiment, pilot estimation results are low pass filtered signals of ontime symbol signals which are normally generated upstream of the tracking circuit. Thus, with this invention, additional structure is not required to obtain improved tracking results.
The foregoing has outlined, rather broadly, the preferred feature of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. While the present invention is embodied in hardware, alternate equivalent embodiments may employ, whether in whole or in part, firmware and software. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.